1. Field of the Invention
The present invention relates to a radio communication system, a base transceiver station, and an extension device.
2. Description of the Related Art
FIG. 17 is a schematic of a conventional mobile radio communication system. As shown in FIG. 17, in the conventional system, a plurality of radio network controllers (RNC) 2 are connected to a core network (CN) 1. A plurality of base transceiver stations (BTS) 3 are connected to each of the radio network controllers 2. Each of the base transceiver stations 3 is disposed in each service area 4.
An optical extension device 6 is disposed in areas 5 that cannot be covered by an ordinary base transceiver station 3. The optical extension device 6 has a parent-child relationship with existing base transceiver stations 3. The optical extension device 6 performs radio communication with a mobile station (user equipment (UE), also referred to as a mobile terminal). A wide transmission bandwidth is required between the base transceiver station 3 and the optical extension device 6, controlled by the base transceiver station 3. Therefore, the base transceiver station 3 and the optical extension device 6 are interconnected by an optical cable.
When the optical extension devices 6 are connected to the base transceiver station 3 that is the parent, the optical extension devices 6 that have the same base transceiver station 3 as the parent can share resources of the base transceiver station 3 that is the parent. However, the optical extension devices 6 cannot share the resources of another base transceiver station 3 that is not the parent. When the optical extension devices 6 are not connected, the resources of the base transceiver station 3 are fixed to the base transceiver station 3 and are not shared with other base transceiver stations 3.
FIG. 18 is a schematic of a base transceiver station and an optical extension device in a conventional mobile radio communication system. The conventional base transceiver station 3 includes an interface unit (IF) 11, a base band processing unit (BB) 12, a spreading processing unit (SP) 13, a receiving unit (RX) 14, an optical interface unit (IF) 15, and a control unit 16. The interface unit 11 is for the radio network controller 2. The base band processing unit 12 performs an encoding process on transmission data and a reverse-spreading and decoding process on reception data. The spreading processing unit 13 performs a spreading process on the transmission data. The receiving unit 14 performs a reception process on the reception data. The optical interface unit 15 is for the optical extension device 6. The control unit 16 controls the overall base transceiver station 3.
The optical extension device 6 includes an optical interface (IF) 17 and a radio transceiving unit (TRX) 18. The optical interface 17 is for the base transceiver station 3. In other words, in the conventional system, spreading-processed data is transmitted between the base transceiver station 3 and the optical extension device 6. As a result, the transmission bandwidth between the base transceiver station 3 and the optical extension device 6 is widened.
The base band processing unit 12 is provided in the form of a card (hereinafter, “BB card”) that can be attached to and removed from the base transceiver station 3. One BB card has a plurality of resources. Each resource is assigned by a channel. Each terminal uses the resources in the base transceiver station 3 and is connected to the core network 1. A sufficient number of BB cards to process a maximum number of terminals expected to be connected is mounted onto each base transceiver station 3.
Plural cells are assigned to each base transceiver station 3. A shared channel used in each cell is assigned to the resources of separate BB cards for each cell, and the assignment is fixed. At the same time, assignment of an individual channel used by each terminal is not limited to the BB card to which the shared channel of the cell, in which the terminal is present, is assigned. The individual channel can be assigned to the resources of a BB card to which the shared channel of another cell of the same base transceiver station 3 is assigned.
Generally, the total number of the resources in each base transceiver station 3 is less than a theoretical maximum storage capacity of one cell (about 80 resources, determined by signature and taking into consideration handovers from other cells). The control unit 16 performs processing, such as resource management and the assignment of the resource to the individual channel, based on instructions from the radio network controller 2.
According to the third generation partnership project (3GPP), various specifications related to a third generation mobile phone system are standardized. For example, specifications regarding a wireless interface between a mobile terminal and a universal mobile telecommunications system (UMTS) terrestrial radio access network (UTRAN), including the radio network controller 2 and the base transceiver station 3 that is controlled by the radio network controller 2, specifications regarding information transmitted between the radio network controller 2 and another radio network controller 2 or another system, specifications regarding an interface between the UTRAN and the core network 1, and specifications regarding an interface between the radio network controller 2 and the base transceiver station 3 are standardized (for example, “3GGP Release 5 Technical Specifications (TS) TS25.331, TS25.413, TS25.433”, [online], The 3rd Generation Partnership Project (3GPP), [searched Dec. 28, 2005], the Internet URL: http://www.3gpp.org/ftp/Specs/html-info/25-series.htm).
In the conventional base station system, the resources are not shared among the base transceiver stations. Therefore, the resources cannot be flexibly assigned according to increases and decreases in traffic varied by time, day of the week, month, season, or the like, and sudden increases in traffic due to an event, a disaster, or the like. As a result, the base transceiver station cannot perform a requested call processing when the resources of the base transceiver station are insufficient for the increase in traffic, thereby leading to a reduction in service quality.
The increase in traffic can be handled by an addition of the BB cards to the base transceiver station with the increased traffic. However, an administrator is required to constantly monitor fluctuations in the traffic and to perform BB card addition operations, thereby leading to an increase in communication costs. At the same time, when the traffic decreases or an actual amount of traffic is less than an expected amount of traffic, operational efficiency of the base transceiver station decreases because the resources of the base transceiver station are not used and become redundant. This also leads to the increase in communication costs.
The resources of the base transceiver station can be assigned with some degree of flexibility in response to the increases and decreases in traffic, by the placement of the optical extension device. However, an optical cable is used to connect the base transceiver station and the optical extension device. Therefore, an area in which the optical extension device can be disposed is limited to an area in which the optical cable can be laid. Moreover, expenses are required to manage the optical cable, thereby leading to the increase in communication costs.